docs: def eccentricity(self):
          """
          The eccentricity of the 2D Gaussian function that has the same
          second-order moments as the source.
  
          The eccentricity is the fraction of the distance along the
          semimajor axis at which the focus lies.
  
          .. math:: e = \\sqrt{1 - \\frac{b^2}{a^2}}
  
          where :math:`a` and :math:`b` are the lengths of the semimajor
          and semiminor axes, respectively.
          """
  
          l1, l2 = self.covariance_eigvals
          if l1 == 0:
              return 0.  # pragma: no cover
          return np.sqrt(1. - (l2 / l1))